1. Field of the Invention
This invention relates to variable magnification optical systems having beam splitters, and more particularly to an arrangement of a view finder or an optical apparatus, such as light emitter or receptor, for automatic focusing adjustment on the path of a split-off part of a light beam.
2. Description of the Prior Art
In the field of 8mm cine cameras, or video cameras, as the size of the effective area of the picture frame is relatively small, it is customary for a beam splitter to take its place in one of the air separations between the members of the photographic lens system so that a light beam toward the film is split into two parts, of which the split-off part goes to a view finder system.
Taking the example of a zoom lens as the photographic lens system, comprising, from front to rear, a first or focusing lens unit stationary during zooming, a second lens unit as the variator, a third lens unit as the compensator, a fourth lens unit from which an afocal beam emerges, and a fixed fifth lens unit, the aperture stop for limiting the light beam and the beam splitter are, in many cases, put in the space between the fourth and fifth lens units.
As far as such a zoom lens for a relatively small effective picture frame is concerned, despite the aperture stop being held stationary during zooming, it is even possible for the beam splitter to be placed near the aperture stop while permitting the beam splitter and the finder system to be of relatively small size.
In the field of 35 mm cameras of the TTL (Through-The-Lens), however, it is customary, particularly in the standard objective, that the aperture stop is made to axially move with zooming. Otherwise, minimization of the bulk and size of the objective could not be achieved. This is exemplified in Japanese Laid-Open Patent Application No. SHO 57-20713.
In application of the aforesaid splitter arrangement to conventional zoom lenses for the 35mm format, therefore, it results that as zooming goes in either direction, the aperture stop moves away from the beam splitter. For this reason, the beam splitter, in the form of a glass block having a half-reflection mirror inclined therein, is necessarily of very large axial thickness to admit even the oblique beam from the margin of the prescribed angular field, and further, the effective diameter of the finder system must also be increased. Thus, the use of the prior known arrangement of the beam splitter and finder system has tended to increase the bulk and size the entire system of the zoom lens.
As to the automatic focusing adjustment of the optical system, in many cameras including both photographic and video cameras, light from an emitter in the camera housing is projected, either directly or through a portion of the objective optical system, onto an object to be photographed, and the reflection of the light from the object is sensed by a receptor in the camera housing either directly or through a portion of the optical system. A typical example of the range finder arrangement of such a so-called modified TTL type, in which light passing through a portion of the optical system is used for detecting the in-focus condition and the light source is positioned within the camera body, is shown in, for example, Japanese Laid-Open Patent Application No. SHO 56-165126.
The range finders of the active type described above generally make use of a light source for emitting infrared radiation in combination with a dichroic mirror of high transmittance for the visible spectral range and high reflectance for the infrared range as the beam splitter in one of the air separations between the members of the optical system.
This beam splitter, when the optical system is of the zoom type, is arranged, in most cases, behind the zoom section to remain stationary during zooming. For this reason, as the aperture stop is moved along with the zoom section to improve the aberration correction and to prevent the diameter of the objective from increasing, the resultant variation of the distance from the beam splitter to the aperture stop with zooming gives rise to the following problem. Referring to FIG. 1, the aperture stop B that moves in unison with the zoom section A is shown as having moved farther away from the beam splitter C by zooming, causing the oblique pencil to be eclipsed by the dichroic mirror D of the beam splitter C. To avoid this, the axial thickness of the beam splitter C must be largely increased. As a result, the physical length of the objective will be increased objectionably. Also, because the transmission of light across the dichroic mirror D differs with diffferent image heights, color are unevenly changed in the marginal zone of the picture frame.